The invention generally relates to a circuit arrangement that may be used to generate light pulses.
Circuit arrangements of this type are used in particular as transmitting units for optical sensors. These optical sensors can be embodied as distance sensors, which typically operate based on the light transit time method. For the distance measuring, light pulses are generated with predetermined timing with the aid of the circuit arrangement of the transmitting unit. The transit time of a light pulse to the object and back to the optical sensor is evaluated as a measure of the distance between an object and the optical sensor.
To achieve a highly precise distance measurement, it is necessary to generate sequences of very short light pulses, wherein the pulse duration of a light pulse typically is approximately one nanosecond. Accordingly, light pulses of this type are also required to have extremely short rise times below one nanosecond.
Circuit arrangements of this type for generating the aforementioned short light pulses are provided with laser diodes functioning as electro-optical converters. A charge store, typically a charge capacitor, is connected to this laser diode by way of a switching element, for example, a transistor.
The charge capacitor is discharged by closing the switching element and, in the process, a current pulse is generated, which is then converted in the laser diode to a light pulse.
With ideal components for such a circuit arrangement, the resulting time history for the current pulse would correspond to a discharge of an ideal RC (resistance-capacitance) element. This would mean an infinitely rapid rise time for the current pulse and an exponential decay of the current pulse.
However, during actual operations system-related deviations occur from the ideal time history of such current pulses, wherein these deviations in particular are caused by parasitic inductances in the components used.
A first deviation is that a finite rise time is obtained for the current pulse in place of an infinitely rapid rise time. Furthermore, the time history of the current pulse takes the form known for the discharge of an RLC (resistance-inductance-capacitance) element. Accordingly, post-oscillations occur during the decay of the current pulse. These post-oscillations comprise negative undershoots which are followed by positive overshoots. The negative undershoots polarize the laser diode in non-conducting direction and result in a Zener breakdown of the pn junction for the laser diode, thereby drastically reducing its operating life.
If the positive overshoots of the current pulse exceed a specific amplitude value, additional parasitic light pulses can thus be generated in the laser diode which follow the actual light pulse and result in distorting the distance measurements.